Introduction The ever-increasing demand for ubiquitous wireless access to content is expected to rapidly evolve in the foreseeable future with the proliferation of several cutting-edge applications such as gaming, mobile HDTV, and personalized wireless services. Meeting the stringent quality of service (QoS) requirements of these applications is contingent upon an efficient exploitation of the radio spectrum. The exclusivity of radio spectrum licenses has led to many inefficiencies. For instance, numerous studies conducted by agencies such as the Federal Communications Commission (FCC) in the United States have shown that much of the licensed radio spectrum remains unoccupied for significant periods of time . The presence of such “spectrum holes” in the licensed spectrum has motivated the development of cognitive radio systems that can improve the efficiency of the wireless spectrum usage [13, 27, 44]. Cognitive radios or secondary users (SUs) are unlicensed wireless devices that can intelligently monitor and adapt to their environment and, hence, they are able to share the spectrum with the licensed primary users (PUs), operating whenever the PUs are idle. In order to provide a conflict-free coexistence between licensed PUs and unlicensed SUs, many technical challenges need to be overcome at two key levels: spectrum sensing and spectrum access [4, 27]. Spectrum sensing is the stage during which the SUs need to sense or observe the PU transmission patterns to identify the presence of spectrum holes that can be used for their transmissions. Once such a transmission opportunity is identified, the next step is spectrum access during which the SUs must agree on how to share the available spectrum.
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